Yes you can. But if you do any work on them, iPhoto will automatically give you an auto-rendered jpeg version of it from RAW, which will most likely not be what you want (as opposed to rendering them in Capture NX or even photoShop). You can still use iPhoto to catalog them, but if you want to do some post on nef files, best to right click on the image in iPhoto and select 'show in Finder'. Then you can work on the original.
